Modern hearing assistance devices, such as hearing aids, typically include a digital signal processor in communication with a microphone and receiver. Such designs are adapted to perform a great deal of processing on sounds received by the microphone. These designs can be highly programmable and may use specialized signal processing techniques for acoustic feedback cancellation and a host of other signal processing activities.
Some acoustic feedback cancellation schemes perform quite well, but may still have difficulty in some situations. There are at least two situations when an adaptive LMS filter may not perform enough feedback cancellation, leading to an audible artifact called a “whoop.” The first situation arises from rapid changes in the acoustic feedback path. If the acoustic feedback path characteristics change too fast (by an important magnitude) the LMS adaptive filter algorithm (commonly used in feedback cancellers) might not adapt fast enough to update the cancellation filter to the new parameters to perform cancellation. During the transition period feedback might not be fully compensated, generating temporary feedback oscillation. This occurs for example when the user approaches the phone headset to his/her ear. In some cases the mistuned LMS cancellation filter might even inject some extra feedback to system.
Another situation where the adaptive LMS filter may not work properly to cancel acoustic feedback occurs where the audio system receives a periodic signal for a relatively long period of time. This is because the adaptive LMS cancellation filter is programmed to respond to the periodicity of the input signal itself instead of the feedback signal. This phenomenon may cause initial attenuation of the input signal, and in the worst case the LMS feedback canceller will actually generate feedback instead of cancelling it.
What is needed in the art is a way to correct for acoustic feedback which is robust enough to compensate for rapid changes of the acoustic feedback path and will not attenuate the input signal for relatively long periodic signal inputs.
Accordingly, there is a need in the art for methods and apparatus for improved signal processing, and in particular for improved acoustic feedback cancellation for hearing assistance devices.